Fix Kubernetes endpoints SD for empty subsets (#3660)

* Fix Kubernetes endpoints SD for empty subsets

When an endpoints object has no associated pods (replica scaled to zero
for instance), the endpoints SD should return a target group with no
targets so that the SD manager propagates this information to the scrape
manager.

Fixes #3659

* Don't send nil target groups from the Kubernetes SD

This is to be consistent with the endpoints SD part.

discovery/kubernetes/endpoints.go

@@ -185,10 +185,6 @@ const (
 )
 
 func (e *Endpoints) buildEndpoints(eps *apiv1.Endpoints) *targetgroup.Group {
-	if len(eps.Subsets) == 0 {
-		return nil
-	}
-
 	tg := &targetgroup.Group{
 		Source: endpointsSource(eps),
 	}

discovery/kubernetes/endpoints_test.go

@@ -339,3 +339,32 @@ func TestEndpointsDiscoveryUpdate(t *testing.T) {
 		},
 	}.Run(t)
 }
+
+func TestEndpointsDiscoveryEmptySubsets(t *testing.T) {
+	n, _, eps, _ := makeTestEndpointsDiscovery()
+	eps.GetStore().Add(makeEndpoints())
+
+	k8sDiscoveryTest{
+		discovery: n,
+		afterStart: func() {
+			go func() {
+				eps.Update(&v1.Endpoints{
+					ObjectMeta: metav1.ObjectMeta{
+						Name:      "testendpoints",
+						Namespace: "default",
+					},
+					Subsets: []v1.EndpointSubset{},
+				})
+			}()
+		},
+		expectedRes: []*targetgroup.Group{
+			{
+				Labels: model.LabelSet{
+					"__meta_kubernetes_namespace":      "default",
+					"__meta_kubernetes_endpoints_name": "testendpoints",
+				},
+				Source: "endpoints/default/testendpoints",
+			},
+		},
+	}.Run(t)
+}

discovery/kubernetes/node.go

@@ -60,6 +60,9 @@ func (n *Node) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 
 	// Send target groups for service updates.
 	send := func(tg *targetgroup.Group) {
+		if tg == nil {
+			return
+		}
 		select {
 		case <-ctx.Done():
 		case ch <- []*targetgroup.Group{tg}:

discovery/kubernetes/pod.go

@@ -68,6 +68,9 @@ func (p *Pod) Run(ctx context.Context, ch chan<- []*targetgroup.Group) {
 
 	// Send target groups for pod updates.
 	send := func(tg *targetgroup.Group) {
+		if tg == nil {
+			return
+		}
 		level.Debug(p.logger).Log("msg", "pod update", "tg", fmt.Sprintf("%#v", tg))
 		select {
 		case <-ctx.Done():